A. Technical Field
The present invention relates generally to the field of optics and more particularly to a system and method for alignment of optical components.
B. Background of the Invention
The importance of aligning optical components within an optical path is well known. An optical path may include various components such as an optical transmitter, fiber optic cable and an optical receiver or detector. The optical transmitter generates and transmits an optical signal using a light source, such as a laser, and corresponding control circuitry. This light source may take the form of a light emitting diode, solid-state laser diode, or other type of light source to generate the optical signal.
The optical transmitter is coupled to optical fiber in which the optical signal propagates. The optical fiber consists of one or more glass fibers, which act as wave-guides for the optical signal. The optical fiber needs to be appropriately aligned or placed in very close proximity to the light-emitting region of the source in order to couple as much light as possible into the fiber. Typically, an optical subassembly or lens barrel is used to couple a light source to a fiber optic cable.
Light sources may be aligned to two different types of optical fiber, which are classified as single-mode fibers and multi-mode fibers. Single-mode fiber is designed with a relatively narrow core diameter, through which only one mode of light propagates. Multi-mode fiber carries numerous modes or light rays simultaneously. Accordingly, this fiber type has a much larger core diameter, allowing for the larger number of modes. The lower order modes, having small propagation angles, strike at or near the center of the fiber, and tend to travel straight down the fiber. On the other hand, higher modes, which do not propagate in the center of the fiber, travel a longer optical path length and thus appear have comparatively low speed of propagation. This is known in the industry as modal dispersion in an optical fiber.
The optical receiver or the detector receives the optical signal from the optical fiber and converts the optical signal back into the original electrical signal. This detector may be mounted in an optical subassembly, similar to the one used for the transmitting optical subassembly, which couples to an optical fiber. Detectors usually have a preferred detecting region and power range that can vary according to the type of detector used.
An optical power may be measured that represents an intensity of light provided by a light source and/or optical fiber at an optical detector. It is desirable to provide the optical signal within a desired power range that is optimal for the particular detector. Optical detectors typically have a pre-defined power range in which the optical detection may be accurately performed. If the power level should drop out of this range, then the detector's performance will likely suffer. Furthermore, optical detectors may be sensitive to the location in which an optical signal strikes the detector. Depending on the characteristics of the fiber (including the type of fiber) and the coupling between the light source and the fiber, an optical signal may be received at various physical locations on a detector.
Encircled flux describes the power encircled within a circular radius inside the optical fiber and denotes the laser flux energy that is coupled into the fiber. Encircled flux is obtained from the near-field intensity pattern produced by a laser source in a graded-index multimode fiber. As the coupling between a light source and a fiber changes, the flux within a circular radius inside the optical fiber may change significantly. Furthermore, a change in coupling or fiber characteristics may result in a change to a corresponding encircled flux value without necessarily changing the effective coupling power.
In current alignment systems, encircled flux is not simultaneously monitored with effective coupling power. Typically, an alignment is performed relative to effective coupling power. After the power alignment is complete, the light source and coupled fiber may be analyzed to relative to encircled flux using a completely different physical measurement on the previously power-aligned light source and coupled fiber.